5.2 Igneous Rocks

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Vocabulary

Extrusive Rock: Igneous rock made from hardened lava
Intrusive Rock: Hardened magma beneath the Earth's surface
Porphyritic texture: Large crystals scattered with a background of smaller crystals

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Outline

Characteristics of Igneous Rock

• Igneous rocks were the first rocks that hardened on the Earth
  • Igneous comes from the Latin word ignis meaning fire
  • Igneous rocks are classified by their origin, texture, and minerals
• Origin
  • There are two classifications of igneous rocks
    • Extrusive rock
      • Basalt is the most common of the extrusive rocks
        • Basalt makes up most of the crust, including lava plateaus, oceanic crust, and shield volcanoes
    • Intrusive rock
      • Granite is the most common intrusive rock
        • Granite batholiths are the core of many dead volcanoes
• Texture
  • An igneous rock's texture are usually fine or coarse grained, or glassy or porphyritic
  • Intrusive and extrusive have different textures
    • Intrusive rocks have a porphyritic texture or larger crystals in front of smaller ones
    • Extrusive rocks have a glassy texture and it contains crystals too small to be seen
• Mineral Composition
  • Magma, high in silica, create granite which can have many different shades
    • Granite, rich in hornblende and mica, is light gray with dark specks while granite rich in feldspar is speckled pink

Uses of Igneous Rocks

• Since igneous rocks are hard, dense, and durable, people used this rock for tools and other building materials.
  • Native Americans have used obsidian for making very sharp tools
  • Granite has been used for more than 3,500 years.
    • Egyptians used it for statues
    • The Incas used granite and other igneous rocks to make a fortress
    • Even today, granite is used to make buildings and bridges
  • Pumice, basalt and obsidian have been also been used for important purposes
    • Crushed basalt is used to make gravel
    • Pumice's rough surface is used for polishing
    • Perlite is mixed in soil for starting vegetable seeds